1. Field of the Invention
The present invention relates to a toner for developing an electrostatic latent image, a developer for developing an electrostatic latent image, a developer unit, and a method for forming an image, which are used in printing using an electrophotography, an electrostatic recording method and the like.
2. Description of the Related Art
Electrophotography, which is a method for visualizing image information by forming an electrostatic latent image, is currently employed in various fields. This method is already well known in the art as described in for instance, U.S. Pat. No. 2,297,691, U.S. Pat. No. 2,357,809 and the like. In general, electrophotography comprises the steps of forming an electrostatic latent image on a photoreceptor by charging and exposing, forming a toner image by developing the electrostatic latent image using a developer containing a toner, transferring the toner image onto a transfer material such as paper or a sheet, and fixing the toner image on the transfer material by utilizing heat, a solvent, pressure and the like to obtain a permanent image.
Recently, electrophotography at present employ a so-called full-color machine in which black toner is used in addition to the magenta, cyan, and yellow toners.
Electrophotography is presently used not only in printers or copiers, but also in a wider application field. In particular, electrophotography is required to be used in a field, which is close to a printing machine, a duplicator and the like. However, so-called full-color machines are generally intended for use in office. Accordingly, such full-color machines are not always applicable to all types of printing field. For instance, such machines cannot be used for all possible printed articles ranging from those paper recording media, such as advertisement leafs, books, magazines, etc., to display plate recording media other than paper, such as electric appliances and automobile bodies. Further, although printing in the field of a duplicator or the like is made using special colors, and there is a strong demand for white color and highly dense red colors, it is difficult for a general full-color machine to form those colors. Moreover, in general, a full-color machine such as full-color printers and full-color copiers cannot satisfy the demand for colors with high image density and high shielding properties.
The demand for high image density and high shielding properties may be satisfied by using a colorant corresponding to the special colors, and by increasing the concentration of the colorant. For instance, a white-colored colorant represented by titanium oxide is generally used for the white toner. However, the toner colorant must be added at a considerably larger amount to achieve the desired high image density and shielding property. Titanium oxide added in a large quantity brings about unfavorable results such as a drop in charge ability, adhesion of the colorants to the carriers and generation of fogging due to an increase in the electric conductivity of the toner. The same tendency occurs with red color toner or with a black color toner.
As the development method of the developer, there are, for instance, a jumping brush development method in which development is carried out while applying a bias electric field between a developer holding member and a photoreceptor, a cascade development method as described in U.S. Pat. No. 2,618,552, a magnetic brush method as described in U.S. Pat. No. 2,874,063, and a touch down method as described in U.S. Pat. No. 2,895,847 and the like. The magnetic brush method is a representative method using a so-called two-component developer prepared by mixing a carrier and a toner. This method uses magnetic particles, such as those of steel and ferrite, as the carriers, and the developer comprising the toner and the magnetic carrier is carried on a magnet, and the developer is formed into a brush-like manner by applying a magnetic field with the magnet. Then, by bringing the magnetic brush into contact with the electrostatic latent image provided on the photoreceptor, the toner inside the brush is attracted to the latent image depending on the quantity of charge to implement the development. The carriers for use in this method can be roughly divided into two types: coated carriers having a coating film on the surface thereof, and non-coated carriers having no coating film on the surface thereof. Many types of coated carriers are developed and brought into practice because the coated carriers are superior with respect to the life of the developer and the like. The coated carriers should at least be capable of providing proper charge ability (charge value and distribution of charge) to the toner, maintaining the proper charge ability for a long duration of time, etc. Accordingly, various types of coated carriers having excellent resistances against impact and friction, which do not change the charge ability of the toner, and which yet remain stable under environmental changes such as those of moisture, temperature, etc has been proposed. For instance, Japanese Patent Application Laid-Open Nos. 61-80161, 61-80162, and 61-80163 disclose preparing a coated carrier having a relatively long life, wherein the coated carrier is prepared by coating the surface of a carrier core material with a copolymer of a nitrogen-containing fluorinated alkyl(meth)acrylate and a vinyl monomer, or a copolymer of a fluorinated alkyl(meth)acrylate and a nitrogen-containing vinyl monomer. Further, Japanese Patent Application Laid-Open Nos. 1-18150 and 2-79862 disclose a coated carrier having a relatively hard coated film, which are each prepared by coating the surface of a carrier core material with a polyamide resin and a melamine resin, respectively, followed by hardening.
However, these carriers have problems in that they cannot sufficiently prevent contamination (impaction) of the carrier surface attributed to the toner components. The contamination of carrier caused by an adhesion of components of a toner to carrier or the like is unpreferable.
In order to prevent impaction from occurring, it has been proposed to form a coated film of a resin having small surface energies on the surface of carriers, for instance, the silicone resin is disclosed in Japanese Patent Application Laid-Open No. 60-186844, or the fluororesin disclosed in Japanese Patent Application Laid-Open No. 64-13560. However, in the carriers above, the silicone resin or the fluororesin certainly exists relatively in a large amount in the vicinity of the carrier surface, but thickness of the coated film is small. Accordingly, the effect of the resin on these carriers is gradually lost, and, inversely, the problem of the impaction is caused after prolonged use due to wear of the coated film, which is gradually caused. Furthermore, in case of performing copying continuously using the developer as described above, it is certainly possible to produce an initial image with excellent reproducibility in image density and image quality. However, after producing several ten thousands of copies, the image density decreases to reproduce an image of poor gradation and granularity. In particular, when toner wherein the concentration of colorants is increased is used to obtain high image density, especially in the case of white toners using titanium oxide in a large amount is used, image degradation occurs due to impaction occurring on the carrier surface.
On the other hand, in order to match processes within the copier, the toner used therein should be superior in flow ability, resistance against caking, fixing ability, charge ability, ability to be cleaned and the like. Accordingly, it has recently been proposed to add a highly value-added additive capable of imparting favorable flow ability, resistance against caking, maintainability of charge, environmental stability and the like as an external additive. For instance, it has been proposed to add an inorganic oxide, such as silica, titanium oxide on the surface of the toner.
Silica exhibits excellent properties such as resistance against caking and the ability to impart flow ability to the toner. However, due to its strong negative charge characteristics intrinsic to the silica itself, it tends to provide a charging ability depending on the environmental conditions. Accordingly, various proposals have been made to apply a hydrophobic treatment to the surface of silica (see, for instance, Japanese Patent Application Laid-Open Nos. 46-5782, 48-47345, 48-47346, 64-73354 and 1-237561 etc.). However, the environmental dependence of charging ability of silica has not been sufficiently improved up to the present without negatively influencing other characteristics such as the resistance against caking and flow ability of silica.
When titanium oxide is added to the toner surface from exterior, the charge level can be lowered. Further, when titanium oxide is applied a hydrophobic treatment with applying a hydrophobic treatment agent and added to a toner from exterior, the charge level and the environmental dependence of the toner can be more easily controlled. However, on the other hand, it suffers problems concerning aggregation of titanium oxide and the like after performing the hydrophobic treatment. Accordingly, since the amount to be subjected to the hydrophobic treatment becomes limited, the charge level cannot be decreased over a certain value. Titanium oxide described above can be produced, in general, by refining and calcining TiO(OH)2 obtained by sulfuric acid method (wet method) with ilmenite ores. Titanium oxide obtained in accordance with the process above certainly contains aggregated particles that are generated as a result of dehydration condensation. Re-dispersion of such aggregated particles is, however, not easy by using a prior art technology. When a crystalline titanium oxide (i.e., rutile having a specific gravity of 4.2 or anatase having a specific gravity of 3.9) is prepared into fine powder, secondary aggregation or tertiary aggregation is found to occur in the powder. This considerably impairs the effect of improving flow ability of the toner as compared with that of silica. Concerning the recent market demand for high quality images, in particular, the trend is for decreasing diameter size of toner particles. However, toner particles with smaller diameter increase the adhesion strength among the particles as to impair the flow ability of the toner. Hence, when toner particles with smaller diameter are used, the flow ability-imparting ability of titanium oxide further tends to become inferior.
In light of such circumstances, it has been proposed to add hydrophobic titanium oxide conjointly with hydrophobic silica to the toner, both of which being obtained by applying thereto surface treatment, with an aim to achieve both improvements in flow ability and in the environmental dependence of charging (see Japanese Patent Application Laid-Open No. 60-136755, etc.). However, due to stress applied by stirring, the effect of only one of the two tends to be exhibited more distinctly. Hence, it is difficult to mutually compensate for the disadvantages over a long period of time. For instance, the hydrophobic titanium oxide improves the charge characteristics and the flow ability of the toner at the initial stage. However, the collision of the toner with the carrier caused by stirring, or the friction between the toner and the blade or the sleeve causes a coupling agent to peel off from the surface of the titanium oxide as to greatly change the charge characteristics of the toner. The reason for this is that, presumably, in case of the hydrophobic titanium oxide described above, the reactivity of core surface of the titanium oxide with respect to the coupling agent is very weak, and that thereby the bonding with the coupling agent becomes extremely weak as compared with the case of silica. Furthermore, due to the embedding of the additives into the surface of the toner, there occurs another problem in that the flow ability of the powder is impaired, and that contamination (spent) by these additives is generated on the surface of the carriers.
On the other hand, it has been proposed to add a hydrophobic amorphous titanium oxide to the toner from exterior (see Japanese Patent Application Laid-Open Nos. 5-204183 and 5-72797, etc.). In this case, however, because the adhesion force of the amorphous titanium oxide with respect to the photoreceptor is too strong, there are problems such as flaws on the photoreceptor on cleaning and white spots on images.